Small renewable energy harvesting and power generation technologies (such as solar arrays, wind turbines, micro sterling engines, and solid oxide fuel cells) are proliferating, and there is a commensurate strong need for intermediate size secondary (rechargeable) energy storage capability. Energy storage batteries for these stationary applications typically store between 1 and 50 kWh of energy (depending on the application) and have historically been based on the lead-acid (Pb acid) chemistry. The batteries typically comprise a number of individual cells connected in series and parallel to obtain the desired system capacity and bus voltage.
For vehicular and stationary storage applications, it is not unusual to have batteries with bus voltages in the hundreds or thousands of volts, depending on application. In these cases, where many units are connected electrically in series, there is typically an inherent need for these cells to be as similar to each other as possible. In the event that the cells are not similar enough, a cell-level monitoring and controlling circuit is commonly necessary. If some set of cells in a string of cells have lower charge capacity than others in the same string, the lower capacity cells will reach an overcharge/undercharge condition during full discharge or charge of the string. These lower capacity cells will be de-stabilized (typically due to electrolyte corrosion reactions), resulting in diminished lifetime performance of the battery. This effect is common in many battery chemistries and is seen prominently in the Li-ion battery and in the supercapacitor pack. In these systems, costly and intricate cell-level management systems are needed if the cells are not produced to exacting (and expensive) precision.